Turbochargers are typically used to increase the power output of an internal combustion engine such as in an automobile or other vehicle. A conventional turbocharger includes a turbine and a compressor. The turbine is rotatably driven by the exhaust gas from the engine. A shaft connects the turbine to the compressor and thereby rotates the compressor. As the compressor rotates, it compresses air that is then delivered to the engine as intake air. The increase in pressure of the intake air increases the power output of the engine. In a typical turbocharger for an internal combustion engine of an automobile, the compressor is a centrifugal compressor, i.e., air enters the compressor in a generally axial direction and exits the compressor in a generally radial direction.
Compressor surge refers to a generally undesirable operating condition in which the flow begins to separate on the compressor blades because of excessive incidence angle. Surge typically occurs when the compressor is operated with a relatively high pressure ratio and with low flow therethrough. For example, compressor surge can occur when the engine is operating at high load or torque and low engine speed, or when the engine is operating at a low engine speed with a high rate of exhaust gas recirculation from the engine exhaust side to the intake side. Compressor surge can also occur when a relatively high specific power output, e.g., more than about 70 to 80 kilowatts per liter, is required of an engine with an electrically assisted turbocharger. Additionally, surge can occur when a quick boosting response is required using an electrically assisted turbocharger and/or variable nozzle turbine (VNT) turbocharger, or when the engine is suddenly decelerated, e.g., if the throttle valve is closed while shifting between gears.
As a result of any of the foregoing operating conditions, the compressor can surge as the axial component of absolute flow velocity entering the compressor is, low in comparison to the blade tip speed in the tangential direction, thus resulting in the blades of the compressor operating at a high incidence angle, which leads to flow separation and/or stalling of the blades. Compressor surge can cause severe aerodynamic fluctuation in the compressor, increase the noise of the compressor, and reduce the efficiency of the compressor. In some cases, compressor surge can result in damage to the engine or its intake pipe system.
JP Publication No. 09310699 discloses a centrifugal compressor in which a plurality of slits 16 pierce a periphery of a suction part casing 11 for partially supplying discharge fluid to an air reservoir 17 through a flow control valve 19. Fluid flowing to an impeller 13 is previously whirled round to decrease the angle of attack at low flow to prevent surging. The flow control valve 19 can be closed if the flow therethrough is not needed to prevent losses.
DE 102 23 876 A1 illustrates a turbocharger in which air can be directed from the volute to the inlet. For example, as shown in FIGS. 4 and 5, air can flow from the volute 21 via member 31 and passage 29 to the compressor inlet.
U.S. Pat. No. 2,656,096 to Schwarz, directed to a centrifugal pump and compressor, discloses that exhaust ports are provided in the side walls of a diffuser. The exhaust ports can be connected to the suction pipe or intake of the apparatus so that the fluid of the boundary layers in the diffuser is sucked out from the diffuser, and the fluid can used to cool the blades and wheels of a turbine.
DE 198 23 274 C1 illustrates a turbocharger that also provides for circulation of gas from the volute to the intake. For example, as shown in the figures, a passage 4 can connect the volute to the intake so that air flows in the indicated direction from the volute to the intake.
Nevertheless, there exists a need for an improved apparatus and method for providing compressed gas, such as in a turbocharger, while reducing the occurrence of compressor surge. In some cases, the prevention of compressor surge can expand the useful operating range of the compressor.